Antiferromagnetism in the Face-Centered Cubic Lattice. III.α-MnS

Alpha MnS is a face-centered cubic antiferromagnet which exhibits the same ferromagnetic-layer spin arrangement which is found in MnO. The Mn⁵⁵ zero-field nuclear magnetic resonance (NMR) has been observed in the antiferromagnetic state of α-MnS and, from the temperature dependence of the resonance frequency, information is derived regarding the spin deviation as a function of temperature. The so-called anomalous temperature dependence of sublattice magnetization which has been reported for MnO is found to be essentially absent in α-MnS. The general question of the incorporation of biquadratic exchange terms -j(Sᵢ·Sⱼ)² into molecular-field and spin-wave theories of magnetism is discussed, and a simple spin-wave theory is developed for α-MnS which includes both bilinear and biquadratic exchange interactions between nearest and next-nearest neighbors (J₁, j₁; J₂, j₂) as well as both isotropic and anisotropic magnetostrictive terms. The spin deviation is found to be an extremely sensitive function of j₁ and of distortion (anisotropic magnetostriction), but is relatively insensitive to j₂ and isotropic magnetostriction. The bilinear exchange interactions are estimated by using Lines' Green's-function theory to describe the paramagnetic properties of α-MnS, for which we find values J₁=7^∘K and J₂=12.5^∘K. Comparison of theory with experiment for the temperature dependence of spin deviation allows an upper limit to be placed upon the magnitude of j₁. The ratio j₁J₁ is found to be less than 10^-3, which is at least an order of magnitude smaller than any previous estimates made for Mn²⁺ interactions.